AC/DC multi-axis accelerometer for determining patient activity and body position

ABSTRACT

An implantable cardiac stimulation device which determines stimulation based upon the patient&#39;s body position and activity level while eliminating special implantation or calibration procedures. To eliminate such special implantation and calibration procedures, the stimulation device correlates the patient&#39;s body position using a multi-axis DC accelerometer or other sensor during times of high activity and determines a patient&#39;s standing position value. During other times, the stimulation device compares the signals from the accelerometer to the standing position value to determine the patient&#39;s current body position. Based upon the current body position and the activity level, the stimulation device determines the necessary stimulation to deliver to the patient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/457,451, filed Dec. 8, 1999, now U.S. Pat. No. 6,466,821.

FIELD OF INVENTION

This invention relates to cardiac stimulators which use indicators ofpatient activity and body position to determine the type and intensityof cardiac stimulation.

BACKGROUND OF THE INVENTION

The class of medical devices known as cardiac stimulation devices candeliver and/or receive electrical energy from the cardiac tissue inorder to prevent or end life debilitating and life threatening cardiacarrhythmias. Pacing delivers relatively low electrical stimulationpulses to cardiac tissue to relieve symptoms associated with a slowheart rate, an inconsistent heart rate, an ineffective heart beat, etc.Defibrillation delivers higher electrical stimulation pulses to cardiactissue to prevent or end potentially life threatening cardiacarrhythmias such as ventricular fibrillation, ventricular tachycardia,etc.

Early advances in pacing technology have led to a better quality of lifeand a longer life span. The development of demand pacing, in which thestimulator detects the patient's natural cardiac rhythm to preventstimulation during times which the patient's heart naturally contracts,led to a more natural heart rate as well as a longer battery life.

Another major advance was rate responsive pacing in which the stimulatordetermines the stimulation rate based upon the patient's metabolicdemand to mimic a more natural heart rate. The metabolic demandtypically is indicated by the patient's activity level via a dedicatedactivity sensor, minute ventilation sensor, etc. The stimulator analyzesthe sensor output to determine the corresponding stimulation rate.

A variety of signal processing techniques have been used to process theraw activity sensor output. In one approach, the raw signals arerectified and filtered. Also, the frequency of the highest signal peakscan be monitored. Typically, the end result is a digital signalindicative of the level of sensed activity at a given time. The activitylevel is then applied to a transfer function that defines the pacingrate (also known as the sensor indicated rate) for each possibleactivity level. Attention is drawn to U.S. Pat. No. 5,074,302 to Poore,et al., entitled “Self-Adjusting Rate-Responsive Pacemaker and MethodThereof”, issued Dec. 24, 1991. This patent has a controller thatrelates the patient activity level signal to a corresponding stimulationrate. In addition, the controller uses the activity signal over a longtime period to determine the adjustment of the corresponding stimulationrates. The activity signal can also indicate when a patient is sleepingto modify the pacing rate as set forth in U.S. Pat. No. 5,476,483 toBornzin, et al, entitled “System and Method for Modulating the Base Rateduring Sleep for a Rate-Responsive Cardiac Pacemaker”, issued Dec. 19,1995, which is hereby incorporated by reference in its entirety.

Another method of determining the stimulation rate based upon metabolicneed is based upon the body position of a patient. Studies have shownthat that a patient being upright indicates a higher stimulation ratethan for a patient lying down. An example is U.S. Pat. No. 5,354,317 toAlt, entitled “Apparatus and Method of Cardiac Pacing Responsive toPatient Position”, issued Oct. 11, 1994. In this patent, the controllermonitors a motion sensor to produce a static output which represents theposition of the patient, i.e., lying down or upright. This static outputis used to determine whether a sleep indicated rate or an awake baserate should be used. However, this system is completely dependent uponthe proper orientation of the stimulator housing during implantation forconsistent and reliable results.

To further improve the stimulator's ability to mimic the heart's naturalrhythm, a combination of monitoring both the patient's activity leveland the body position has been envisioned. U.S. Pat. No. 5,593,431 toSheldon, entitled “Medical Service Employing Multiple DC Accelerometersfor Patient Activity and Posture Sensing and Method”, issued Jan. 14,1997, sets forth a system which monitors both parameters. This patentsets forth a cardiac stimulator which uses a multi-axis DC accelerometersystem to monitor both patient position and patient activity.Unfortunately, this accelerometer also depends upon a known orientationduring implant and repeated postoperative calibrations for properoperation due to shifting of the stimulator within the implant pocket.

The ability to accurately determine both the patient's activity leveland the patient's body posture would greatly benefit many patients byproviding a more metabolically correct stimulation rate. As well, thiscombination of sensors could be used to determine the accuracy of othersensors such as PDI, O₂ saturation, etc. This enables the controller ofthe stimulator to blend the outputs of the various sensors to providethe benefit of the each individual sensor. Also, in the case ofimplantable cardioverter/defibrillators (ICDs), these two outputs wouldallow modification of the defibrillator thresholds based upon time ofday and posture of the patient.

Accordingly, it would be desirable to develop an implantable cardiacstimulator which adjusts the stimulation level based upon the patientactivity and the patient body position via sensors that are not deviceimplant orientation sensitive.

SUMMARY OF THE INVENTION

The present invention is directed towards an implantable cardiacstimulation device which determines cardiac stimulation levels basedupon the patient's current body position and activity level whileeliminating special implantation or calibration procedures. To determinethe body position and the activity level, the stimulator monitors theoutput of a multi-axis DC accelerometer or a combination of sensors toinclude oxygen saturation, PDI, minute ventilation sensors, etc.

To determine the patient's current body position, the controllerestablishes the output of at least two DC accelerometers during times ofhigh activity as the patient's standing position. Lower activity levelsassociated with the other body orientations while lying down are alsodeduced in a similar manner. Then, the stimulator correlates the currentoutputs of the DC accelerometers with the standing position to determinethe current body position and uses the previous and current bodypositions and the activity level (preferably calculated from the ACacceleration in the anterior-posterior axis, the axis which has the bestcorrelation with the patient activity), to determine the instantaneousstimulation needed.

Because this stimulator depends upon the combination of activity signalsand position signals from the multi-axis accelerometer, this device isnot dependent upon a predetermined implant orientation or repeatedcalibration of the accelerometer after implant. If the device shouldshift within the patient after implant, the controller will accommodatethis change during the patient's next high activity period.

As such, this device monitors the activity and position signals from themulti-axis accelerometer to determine the indicated activity level ofthe patient and the current body position and then determines the typeand intensity of cardiac stimulation the patient needs.

In a further aspect of a preferred embodiment of the present invention,the calculated standing position is monitored to detect changes that mayindicate the presence of twiddler's syndrome.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 is a block diagram of an implantable stimulation device as setforth in the present invention;

FIG. 2 is an example of an exemplary three-axis accelerometer within astimulator housing, suitable for use with the present invention;

FIG. 3 sets forth a two-dimensional cluster plot as outputted by theaccelerometer;

FIG. 4 sets forth a flow chart for a method of determining the variables(x_(c), y_(c)) in accordance with the present invention;

FIGS. 5A–5B set forth a flow chart for a method for determining thecurrent position of the patient in accordance with the presentinvention;

FIG. 6 sets forth a transfer function for determining the activityindicated rate;

FIG. 7 sets forth a transfer function for determining the circadian baserate;

FIG. 8 sets forth a normal heart rate response when a patient standsafter a prolonged period of lying down;

FIG. 9 sets forth a time table of an example of an orthostaticcompensation stimulation regime;

FIG. 10 sets forth a flow chart for a method of determining aninstantaneous stimulation rate in accordance with the present invention;and

FIG. 11 set forth a flow chart of an exemplary supplement to the flowchart of FIG. 4 for detecting twiddler's syndrome.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, an implantable cardiac stimulation device inaccordance with the present invention is shown as a dual sensor rateresponsive pacemaker 10. It is well within the scope of this inventionto operate this stimulation device in a demand mode as is well knownwithin the art. Also, the principles of this invention would be easilyapplied to defibrillation theory by one of ordinary skill in the art.While the preferred embodiment is directed towards a stimulation devicewhich uses a multi-axis (two or more axis) accelerometer for determiningthe pacing rate and stimulation intensity, it is well within the scopeof this invention to apply the principles of this invention for use withother physiologic sensors that also indicate patient position andactivity.

FIG. 1 sets forth a simplified block diagram of the stimulation device10. The stimulation device 10 is coupled to a heart 5 by way of twoleads 12, 14. The first lead 12 has at least one electrode 18 in contactwith the atrium of the heart 5, and the second lead has at least oneelectrode 20 in contact with the ventricle of the heart 5. The leads 12,14 are electrically and physically connected to the stimulation device10 through a connector 16 which forms an integral part of the housing(not shown) in which the circuits of the stimulation device 10 arehoused. The connector 16 electrically protects circuits within thestimulation device 10 via protection network 17 from excessive shocks orvoltages that could appear on electrodes 18, 20 in the event of contactwith a high voltage signal, e.g., from a defibrillator shock.

The leads 12, 14 carry the stimulating pulses to the electrodes 18, 20from the atrial pulse generator (A-PG) 22 and a ventricular pulsegenerator (V-PG) 24, respectively. Further, the electrical signals fromthe atrium are carried from the electrode 18 through the lead 12 to theinput terminal of the atrial channel sense amplifier (P-AMP) 26. Theelectrical signals from the ventricle are carried from the electrode 20through the lead 14 to the input terminal of the ventricular channelsense amplifier (R-AMP) 28. Similarly, electrical signals from both theatrium and the ventricle are applied to the inputs of the IEGM(intracardiac electrogram) amplifier 30. The stimulation device 10detects an evoked response from the heart to an applied stimulus,allowing the detection of the capture with a suitable broad bandpassfilter. The IEGM amplifier 30 is also used during the transmission to anexternal programmer 60.

The stimulation device 10 uses a controller (microprocessor control andtiming circuits) 32 that typically includes a microprocessor to carryout the control and timing functions. The controller 32 receives outputsignals from the atrial amplifier 26, the ventricular amplifier 28, andthe IEGM amplifier 30 over the signal lines 34, 36, 38, respectively.The controller 32 then generates trigger signals that are sent to theatrial pulse generator 22 and the ventricular pulse generator 24 overthe signal lines 40, 42, respectively.

The stimulation device 10 also includes a memory 48 that is coupled tothe controller 32 over a suitable data/address bus 50. This memory 48stores customized control parameters for the stimulation device'soperation for each individual patient. Further, the data sensed by theIEGM amplifier 30 may be stored in the memory 48 for later retrieval andanalysis.

A clock circuit 52 directs appropriate clock signal(s) to the controller32 as well as any other circuits throughout the stimulation device 10,e.g., to the memory 48 by a clock bus 54.

The stimulation device 10 also has a telemetry communications circuit 56which is connected to the controller 32 by way of a suitablecommand/data bus 58. In turn, the telemetry communications circuit 56 isselectively coupled to the external programmer 60 by an appropriatecommunications link 62, such as an electromagnetic link. Through theexternal programmer 60 and the communications link 62, desired commandsmay be sent to the controller 32. Other data measured within or by thestimulation device 10 such as IEGM data, etc., may be sorted anduploaded to the external programmer 60.

The stimulation device 10 derives its electrical power from a battery 64(or other appropriate power source) which provides all operating powerto all the circuits of the stimulation device 10 via a POWER signal line66.

The stimulation device 10 also includes a sensor 68 that is connected tothe controller 32 over a suitable connection line 72. In the preferredembodiment, this sensor detects patient activity and indicates thepatient's position via a multi-axis (i.e., two or more axis) DCaccelerometer. However, any appropriate sensor or combination of sensorswhich indicate levels of patient activity and indicate the patient'sposition could be used. Other such sensors, such as a minute ventilationsensor, blood pressure sensor, PDI sensor, etc., can be used to in lieuof or supplemental to the activity signal from the accelerometer. In thecase of some of these alternative sensors, the sensor could be placed onthe lead 14 as shown by an alternative sensor 69.

The above described stimulation device 10 generally operates in aconventional manner to provide pacing pulses at a rate that comfortablymeets the patient's metabolic demands. In a typical case, the controller32 uses the signals generated by the sensor 68 (or other alternativesensors 69) to determine both the activity level and the body positionof the patient, both indicators of metabolic need. Many methods ofdetermining the activity level of the patient are well known within theart. Attention is drawn to the '483 patent, which is hereby incorporatedby reference. To regulate the pacing rate, the controller 32 provides atrigger signal to the atrial pulse generator 22 and/or the ventricularpulse generator 24. The timing of this signal (to a large extent) isdetermined by the activity level of the patient, body position, and theindividualized control parameters.

In embodiments of the present invention, each multi-axis DCaccelerometer consists of at least two DC accelerometers (hereafterknown as DC sensors), preferably mounted essentially orthogonal to eachother. FIG. 2 sets forth an exemplary embodiment with three DC sensorsattached to the inside of a stimulator housing 205. In the example ofFIG. 2, these three DC sensors are labeled superior-inferior 210,anterior-posterior 215, and lateral-medial 220, respectively. Each DCsensor can also generate the activity level of the patient, i.e., ACacceleration.

Each of the DC sensors 210, 215, 220 is preferably a surfacemicromachined integrated circuit with signal conditioning as is wellknown in the art. Employing surface micromachining, a set of movablecapacitor plates are formed extending in a pattern from a shapedpolysilicon proof mass suspended by tethers with respect to a furtherset of fixed polysilicon capacitor plates. The proof mass has asensitive axis along which a force between 0 G and +50 G effects thephysical movement of the proof mass and a change in the measuredcapacitance between the fixed and moveable plates. The measuredcapacitance is transformed by the on-chip signal conditioning circuitsinto a low voltage signal. Further information regarding the physicalconstruction of the DC sensors can be found in the '431 patent, herebyincorporated by reference in its entirety. However, many other types ofaccelerometers are commercially available, and it would be obvious toone of ordinary skill in the art to use other types of accelerometers inplace of the above described multi-axis DC accelerometer. While, threeor more DC sensors are preferred, systems of the present invention canbe formed from two DC sensors, as described further below.

To determine both the activity level and the body position of thepatient, the controller 32 monitors the output of each of the DCsensors. Preferably using standard analog to digital conversiontechniques, the output of each of the DC sensors is filtered to separatean AC signal component (representing the activity level) and a DC signalcomponent (representing the body position). Then, the two resultantsignals are further processed to determine two corresponding digitaloutputs which represent the instantaneous signal level of each signal.The resulting activity digital signals are then further processed todetermine the activity level of the patient by methods well known withinthe art. One example is contained in the '302 patent, herebyincorporated by reference in its entirety. The resulting two positionsignals are processed to determine the indicated body position asdiscussed below.

Knowing the activity level, the activity variance measurements can bedetermined. Activity variance is the long term variance in the patient'sactivity as derived by the controller 32 and gives a further indicationof the patient's activity. For example, a high variance measurementindicates the patient has been quite active and a low variancemeasurement indicates that a patient has been resting or sleeping.Preferably, these activity and activity variance signals are calculatedfrom the accelerometer output in the anterior-posterior axis which hasthe best correlation with the patient physical activity. For furtherinformation regarding the determination of the activity variance,attention is drawn to the '483 patent, hereby incorporated by referencein its entirety.

Once the activity level of the patient is determined, the controller 32then determines the instantaneous position of the stimulator housing 205within the patient. By using the digital outputs of the lateral-medial220 and superior-inferior 210 DC sensors shown in FIG. 2, a twodimensional plot can be created to show the clusters in the differentgraph positions which represent different body postures of the patient.When the stimulator housing 205 shifts within the patient's body, theclusters on the two dimensional plot will rotate correspondingly, butthe relative positions of the clusters will not change. The graph ofFIG. 3 is defined as follows: x is the current indicated position in thelateral-medial axis, y is the current indicated position in thesuperior-inferior axis, and (x_(c), y_(c)) represents the average of thedigital outputs that have correlated to the patient being oriented in astanding position.

To determine the orientation of the accelerometer indicating that thepatient is standing, attention is drawn to FIG. 4 which shows anexemplary flowchart. First, at step 405, the activity level (LastAV) andthe activity variance (Act_(—)Var) measurements are monitored as well asthe current digital outputs from the DC sensors, e.g., 210, 220. Thedigital outputs are used to determine an initial (x, y) value. Then, atstep 410, both the activity level (LastAV) and the activity variance(Act_(—)Var) measurements are compared to corresponding standingthresholds, respectively Activity Standing Threshold and AV StandingThreshold.

If both variables are above their corresponding standing thresholds, thecontroller 32 continues to step 415, where the controller 32 incrementsa counter and adds the current digital outputs indicative of (x, y) toan accumulated (x, y) value. Then, at step 420, the controller 32determines if the counter has reached its threshold value (e.g., 120counts).

If the controller 32 has not reached its threshold value, the controller32 returns to step 405. If the controller 32 has reached its thresholdvalue, the controller 32 continues to step 425 where the controller 32determines (x_(c), y_(c)) by averaging the accumulated (x, y) value overthe counter period. Afterwards, in step 435, the controller 32 resetsthe counter to zero to prepare for another update. The controller thenreturns to step 405.

As shown in step 430, if one of these variables is not above thecorresponding standing threshold, then the counter and the accumulated(x, y) values are reset to 0, and the controller 32 returns to step 405.

Once the controller 32 has determined what position (x_(c), y_(c))corresponds to the patient standing, the controller 32 then determinesthe current body position. FIGS. 5A–5B sets forth an exemplary flowchart to determine the current body position of the patient. To beginthe process, the controller 32 sets the current position code (CurrentPosition) equal to 1. The current position code is a binary code of 1 or0 which indicates whether the patient is standing (1) or lying down (0).Additionally, the controller 32 sets the time the patient has been atrest, T_(REST), equal to 0, and stores T_(REST) into memory 48 at step501. At step 505, the controller 32 reads the current position code andthe activity level (LastAV) out of the memory 48. The activity level(LastAV) is determined from the digital output of the AC portion of theDC sensor output as discussed above. The controller 32 then determinesthe projection value (P).

The projection value (P) is a numerical indication of the correlationbetween the current body position (x, y) to the determined standingposition (x_(c), y_(c)) as calculated below:$P = \frac{\left( {{x^{*}x_{c}} + {y^{*}y_{c}}} \right)}{\left( {x_{c}^{2} + y_{c}^{2}} \right)}$If the projection value (P) indicates a correlation value of greaterthan a standing threshold, e.g., at least 0.65, the current bodyposition (x, y) is considered to be standing. For example, using thevalues referenced in the discussion of FIG. 3, a projection value (P) of0.968 is calculated. Accordingly, the current (x, y) value shown in theexample of FIG. 3 would be considered to correspond to the patient beingin a standing position.

Once the controller 32 has determined these three values (i.e.,T_(REST), activity level (LastAV), and the projection value (P)), itproceeds to step 510. At step 510, the controller 32 determines if thecurrent position code indicates that the patient was standing. If thecurrent position code 1, the controller 32 proceeds to step 515.

Then, at step 515, the controller 32 determines if the currentprojection value (P) is less than a standing threshold(Standing_(—)Threshold), e.g., 0.65, and if the activity level (LastAV)is below an activity threshold (Act_(—)Avg). If either of theseconditions is not true at step 515, the controller 32 proceeds to step520 where the controller 32 stores the current position code as the lastposition code (Last Position) and sets the current position code equalto 1, indicating that the patient is still standing. The controller 32then returns to step 505.

If both of these conditions are true at step 515, the controller 32proceeds to step 525 where the controller 32 compares the current valueof T_(REST) with a resting threshold (Rest_(—)Enough). If T_(REST) isnot equal to a resting threshold (Rest_(—)Enough), then T_(REST) isincremented at step 530. At step 535, the controller 32 stores thecurrent position code as the last position code and sets the currentposition code equal to 1, indicating that the patient is standing sincethe patient has not been at rest for a sufficient period of time. Thecontroller 32 then returns to step 505.

If T_(REST) is equal to the resting threshold at step 525, then, at step540, the last position code is set equal to the current position codeand the current position code is set to 0, indicating that the patientis no longer standing. Then, the controller 32 returns to step 505.

Returning to step 510, if the current position code was not equal to 1,the controller 32 proceeds to step 545, where the controller 32determines if the projection value (P) is greater than the standingthreshold (e.g., 0.65) and if the activity level (LastAV) is greaterthan the activity threshold (Act_(—)Avg), e.g., indicating that thepatient is exercising. If these conditions are both true at step 545,the controller 32 proceeds to step 550 where T_(REST) is set to 0. Thecontroller 32 then proceeds to step 555 where the last position code isset equal to the current position code and the current position code isset to 1 to indicate that the patient is now standing. The controller 32then returns to step 505.

If either condition is not met at step 545, then the controller 32proceeds to step 560 where the last position code is set equal to thecurrent position code and the current position code is set to 0 toindicate that the patient is still at rest. The controller 32 thenreturns to step 505.

While a two dimensional calculation has been described, one of ordinaryskill would appreciate that this calculation could be expanded to athree dimensional case with the use of three DC sensors, e.g., 220, 210,215 of FIG. 2, respectively indicating the (x, y, z) positions of thepatient's body. In such a case the projection value (P) would becalculated as:$P = \frac{\left( {{x^{*}x_{c}} + {y^{*}y_{c}} + {z^{*}z_{c}}} \right)}{\left( {x_{c}^{2} + y_{c}^{2} + z_{c}^{2}} \right)}$

One application of this method of determining body position is inorthostatic compensation pacing. Patients who suffer from long termdiabetes tend to develop neuropathy from the long term exposure of theirnerves to excessive blood sugar levels. This condition erodes the body'sability to adequately control the heart rate. In particular, thiscondition renders the patient unable to compensate for the dramatic dropin blood pressure upon standing after sitting or lying down for anextended period of time due to an inability of the body to increase theheart rate and constrict the system resistance and capacitance of itsblood vessels.

To overcome this condition, the controller 32 compensates for the changein the patient position by triggering an orthostatic compensation ratewhen the body position, the activity level signal, and the activityvariance indicate a sudden change in the patient's activity after aprolonged period of inactivity. This pacing regime is blended into atraditional transfer function indicated by the activity level andactivity variance measurements as discussed below.

In FIG. 6, the activity indicated rate (AIR) is illustrated. Thetransfer function is used by the controller 32 to correlate the activitylevel measurements shown along the horizontal axis to the activityindicated pacing rates shown along the vertical axis. The controller 32then triggers the appropriate pulse generator 22, 24 at the activityindicated rate. It should be noted that an appropriate transfer functioncan be used based upon individual need. In addition, different modes ofpacing (e.g., DDD, VVI, etc.) can be accommodated by this method.

Two activity levels are noted on the horizontal axis of the transferfunction: a low activity threshold 602 and a high activity threshold604. For activity level measurements above the high activity threshold604, the pacing rate is maintained at a maximum pacing rate 606 asdetermined by the physician. For activity level measurements between thelow activity threshold 602 and the high activity threshold 606, theactivity indicated pacing rate varies according to a programmed transferfunction 608. In this case, the activity indicated pacing rate varieslinearly between a base rate 610 and a maximum pacing rate 606. However,this transition can be programmed to meet the patient's need by thephysician in many different ways as is well known in the art orperiodically adjusted by the controller 32 as set forth in U.S. Pat. No.5,514,162 to Bornzin, et al., entitled “System and Method forAutomatically Determining the Slope of a Transfer Function for aRate-Responsive Cardiac Pacemaker”, issued May 7, 1996, herebyincorporated by reference.

The circadian base rate (CBR), illustrated in FIG. 7, is established bymonitoring the activity variance measurements (also known as the longterm variance in activity) as described more fully in the '483 patent,hereby incorporated by reference in its entirety. The transfer functionis used by the control system to correlate the activity variance levelshown along the horizontal axis to the CBR shown along the verticalaxis. The controller 32 then triggers the appropriate pulse generator22, 24 at the activity indicated rate. Preferably, an appropriatetransfer function can be used based upon individual need. In addition,different modes of pacing, i.e., DDD, VVI, etc.) can be accommodated bythis method.

Two activity variance levels are noted on the horizontal axis of thetransfer function: a low activity variance threshold 652 and a highactivity variance threshold 654. For activity variance levelmeasurements above the high activity variance threshold 654, the CBR ismaintained at a maximum pacing rate 656 as determined by the physician.For activity level measurements between the low activity variancethreshold 652 and the high activity variance threshold 654, the CBRvaries according to a programmed transfer function 658. In this case,the CBR varies linearly between a minimum CBR 660 and a maximum CBR 656.For activity variance level measurements below the low activity variancethreshold 652, the controller 32 sets the CBR as defined by the minimumCBR 660.

A patient's activity level is monitored and activity variancemeasurements are calculated to determine when and how long a patienttypically rests or sleeps. These two terms define a stimulation ratewhich is below the programmed base rate such that the controller 32triggers pacing pulses at a lower pacing rate during sleep. This lowerpacing rate more closely mimics the natural cardiac rhythm exhibitedduring rest or sleep.

FIG. 8 sets forth one sample of an experimentally observed orthostaticresponse of a normal healthy person. Upon standing after a prolongedperiod of sitting or lying down, the normal sinus rate quickly increasesto a peak 702 of approximately 80 to 120 bpm within 10 seconds. Then,the rate slowly decreases to the base rate, typically 70 bpm, in about10 to 20 seconds.

FIG. 9 shows an exemplary orthostatic compensation pacing regime thatcan be delivered to the pacemaker patient upon the detection of aposture change from standing up to lying down. The controller 32increases the orthostatic compensation rate (OSCR) from a base rate 800to a peak 802 in seven cardiac cycles. After the OSCR reaches themaximum, it is maintained at the maximum level for another 7 cardiaccycles. The rate then slowly decreases down to the base rate in about 12cardiac cycles. Preferably, the specific orthostatic compensation pacingregime, including the maximum OSCR, the slopes and the duration, can bevaried and is typically determined by the physician. As such, the regimeset forth in FIG. 9 is only an example.

FIG. 10 sets forth an exemplary flow chart describing how the controller32 determines the immediate pacing rate. Upon initiation at step 900,the controller 32 sets the value of the T_(OSC) counter equal to 0. TheT_(OSC) counter represents the cycle pointer of the orthostaticcompensation pacing regime as set forth on the x-axis of FIG. 9. Thecontroller 32 then proceeds to step 905 where the controller 32determines if the T_(OSC) counter is equal to 0. If this condition isnot true at step 905, then the controller 32 is already within theorthostatic compensation (OSC) pacing regime and proceeds to step 910.The controller 32 then sets the stimulation rate (SIR) to be the maximumof an activity indicated rate (AIR), a circadian base rate (CBR), or anorthostatic compensation rate (OSCR(T_(OSC))). Preferably, this functionOSCR(T_(OSC)) is performed as a look-up function of data within anorthostatic compensation table 916 within the memory 48. The controller32 then proceeds to step 915, where the controller 32 increments theT_(OSC) counter.

If at step 920, the T_(OSC) counter has reached the end of theorthostatic compensation table 916, the controller 32 resets the T_(OSC)counter to 0 at step 925 because the orthostatic compensation regime hasbeen completed. The controller 32 then returns to step 905 to continuethe orthostatic compensation regime. If at step 920, the T_(OSC) counterhas not reached the end of the orthostatic compensation table, then thecontroller 32 proceeds directly to step 905.

Returning to step 905, if the T_(OSC) counter is equal to 0, thecontroller 32 proceeds to step 930. This condition represents that thepatient is not currently receiving the orthostatic compensation regime.The controller 32 then reads the last position and the current positioncodes from the memory 48. The controller 32 then, at step 935,determines if the last position code is equal to 0 and if the currentposition code is equal to 1. If these conditions are not met at step930, then, at step 940, the controller 32 sets the stimulation rate(SIR) to the maximum of the activity indicated rate (AIR) and thecircadian base rate (CBR) as discussed above. If these conditions aremet at step 935, then the patient has just stood up after a period oflying down. In this case, the controller 32 triggers the orthostaticcompensation regime. To this end, the controller 32 proceeds to step 945where the controller 32 sets the T_(OSC) counter to 1 and sets thestimulation rate (SIR) to be equal to the maximum of the activityindicated rate (AIR), the circadian base rate (CBR), and the orthostaticcompensation rate (OSCR(T_(OSC))) as set forth above. The controller 32then returns to step 905 where the orthostatic compensation routine willcontinue through step 910 as previously described.

Once the controller 32 determines the instantaneous stimulation rate(SIR), the controller 32 returns to the posture detection mode (seeFIGS. 4 and 5) to determine the new current patient position. Once thecontroller 32 updates the last position code with the current positioncode and determines the current patient position, the controller 32 thenreturns to update the instantaneous stimulation rate (SIR). Also, anytime the controller 32 detects that the activity level exceeds thestanding threshold, the controller 32 returns to update the standingcenter (x_(c), y_(c)) Other applications of this invention includeadjustment of stimulation parameters such as the AV delay, the PVARP,the stimulation level, and timing. For example, the AV delay could beshortened when the patient is known to be standing. Also, the base ratecan be adjusted for a patient being in a standing position versus lyingdown.

Also, knowing the position and the activity level of the patient canallow the triggering of monitoring functions under known conditions. Forinstance, when a patient is reliably known to be sleeping (i.e.,inactive and lying down), monitoring functions such as heart sounds,respirations, intrinsic heart sounds, etc., can be measured forlong-term monitoring of cardiovascular function.

As an additional feature, embodiments of the present invention mayinclude the capability to detect twiddler's syndrome. Twiddler'ssyndrome refers to the condition where an implanted cardiac stimulationdevice sits loosely in the pocket in the patient's chest. Consequently,the cardiac stimulation device may rotate in the pocket due topurposeful or inadvertent activity by the patient, eventually causinglead dislodgment or fracture. By comparing the currently calculatedstanding position (x_(c), y_(c)) to an initial calculation of thestanding position (i.e., its implantation position (x_(ci), y_(ci))),embodiments of the present invention can recognize movements of thecardiac stimulation device that may correspond to twiddler's syndrome.FIG. 11 shows a flow chart of an exemplary supplement to the flow chartof FIG. 4 that may be used for detecting twiddler's syndrome. Thissupplement is placed following block 425 of FIG. 4 where the currentstanding position value (x_(c), y_(c)) is calculated. In step 950, it isdetermined whether this is the first calculation of (x_(c), y_(c)). Ifthis is the first calculation, an initial, e.g., an implantation,standing position value (x_(ci), y_(ci)) is stored in memory 48 and theprocess continues in step 435. Following subsequent calculations of thestanding position, the process continues in step 954 where the currentlycalculated standing position (x_(c), y_(c)) is compared to the initialcalculated standing position (x_(ci), y_(ci)). This calculation may bedone by a linear comparison by separately determining the amount ofchange in the x and y components of the standing position value. Then,in step 956 these x and y values are compared to a twiddler's thresholdvalue to determine if the change in the standing position may indicatetwiddler's syndrome.

Preferably, a calculation may be performed as shown below:$P_{t} = \frac{\left( {{x_{c}^{*}x_{ci}} + {y_{c}^{*}y_{ci}}} \right)}{\left( {x_{ci}^{2} + y_{ci}^{2}} \right)}$to determine the correlation between the initial standing position valueand the current standing position value. (Of course, while a two axescalculation is shown, the calculation can be expanded as previouslydescribed for a three axes embodiment.) A value of less than apredetermined Twiddler's Threshold value, e.g., 0.85, may indicatetwiddler's syndrome. Accordingly, the process continues in step 958where a Twiddler's Flag is set. The process then continues at step 435.

The status of the Twiddler's Flag may be sent to the external programmer60 during a follow up visit of the patient to the physician. Preferably,the setting of the Twiddler's Flag can alert a physician of thepotential prescience of twiddler's syndrome before damage to the leadhas occurred.

The flow chart of FIG. 11 is only an exemplary method of monitoring fortwiddler's syndrome. One of ordinary skill in the art would recognizethat other methods could be used to monitor for trends in the calculatedstanding position. For example, one could monitor for changes in thecalculated standing position between calculations or groups ofcalculations without actually storing the first post implantation value.All such methods are considered to be within the scope of the presentinvention.

Although the invention has been described with reference to particularembodiments, it is to be understood that these embodiments are merelyillustrative of the application of the principles of the invention. Forinstance, this method can also be used to alter the defibrillationparameters in an implantable cardioverter/defibrillator unit.Accordingly, the embodiments described in particular should beconsidered exemplary, not limiting, with respect to the followingclaims.

1. A cardiac stimulation device comprising: means for sensing activitylevel and body position and for generating signals indicative ofactivity level and body position of a patient's body; and a controllerthat is adapted to be coupled to the means for sensing to receive theactivity and position signals, wherein the controller is operative toprocess the activity and position signals to determine a standingposition during an initial phase, and that, in a subsequent phase, isoperative to compare the activity and position signals with thedetermined standing position to determine a patient position.
 2. Thedevice of claim 1, wherein the means for sensing comprises at least onesensor that generates both activity and position signals.
 3. The deviceof claim 1, wherein the means for sensing comprises at least twosensors, wherein at least one of the sensors senses activity level andat least one of the sensors senses body position.
 4. The device of claim1, wherein the means for sensing comprises at least two sensors, whereineach of the at least two sensors is operative to sense activity leveland body position.
 5. The device of claim 1, wherein the controller isfurther operative to determine a stimulation rate based upon activitylevel and body position.
 6. An implantable cardiac stimulation device,comprising: a sensor that senses body position of a patient; a detectorthat detects when the patient is in an active state; a signal processingcircuit, coupled to the sensor and the detector, that automaticallymeasures the patient's body position when the patient is detected in theactive state and generates a standing position value based on themeasured body position; a memory that stores the standing positionvalue; and a controller, coupled to the detector, the memory and thepulse generator, configured to determine a current body position of thepatient relative to the stored standing position value, and furtherconfigured to adapt a stimulation therapy based upon the patient'scurrent body position.
 7. The stimulation device of claim 6, wherein thedetector comprises: an exercise state sensor that senses exercise stateof the patient; and wherein the detector further detects when thepatient's exercise state exceeds the standing threshold.
 8. Thestimulation device of claim 6, wherein the detector comprises: anactivity sensor that senses activity level of the patient; and whereinthe detector further detects when the patient's activity level exceedsthe standing threshold.
 9. The stimulation device of claim 6, wherein:the sensor is configured to sense resting and active states of thepatient; and the detector further detects when the patient is not in theresting state.
 10. The stimulation device of claim 6, wherein: thesensor senses a diurnally varying parameter of the patient and monitorsthe variance of the diurnally varying parameter; and wherein thedetector further detects that the patient is in a wake state when thevariance of the diurnally varying parameter is greater than apredetermined threshold.
 11. The stimulation device of claim 6, furthercomprising: a first sensor positioned along a first sensor axis, x, andconfigured to deliver a first signal having a DC position signalcomponent indicative of the patient's body position along the firstsensor axis and an AC activity level component indicative of activity ofthe patient's body position along the first sensor axis; and a secondsensor positioned along a second sensor axis, y, essentially orthogonalto the first sensor axis, and configured to deliver a second signalhaving a DC position signal component indicative of the patient's bodyposition along the second sensor axis and an AC activity level componentindicative of activity of the patient's body position along the secondsensor axis.
 12. The stimulation device of claim 6, wherein the detectorcorrelates the current body position to the stored standing position viaan equation:$P = \frac{\left( {{x^{*}x_{c}} + {y^{*}y_{c}}} \right)}{\left( {x_{c}^{2} + y_{c}^{2}} \right)}$wherein: x is the current body position signal from the firstaccelerometer along the first sensor axis; y is the current bodyposition signal from the second accelerometer along the second sensoraxis; x_(c) is stored standing position along the first sensor axis; andy_(c) is stored standing position along the second sensor axis.
 13. Thestimulation device of claim 12, wherein the detector is operative todetect that the patient is standing when the projection value (P) isgreater than a predetermined threshold, and that the patient is notstanding when the projection value (P) is less than the predeterminedthreshold.
 14. A cardiac stimulation device comprising: at least onesensor that is operative to sense activity level and body position andfurther operative to generate signals indicative of activity level andbody position of a patient's body; and a controller that is adapted tobe coupled to the at least one sensor to receive the activity andposition signals, wherein the controller is operative to process theactivity and position signals to determine a standing position during aninitial phase, and that, in a subsequent phase, is operative to comparethe activity and position signals with the determined standing positionto determine a patient position.
 15. The device of claim 14, wherein theat least one sensor comprises a single sensor that generates activityand position signals.
 16. The device of claim 14, wherein the at leastone sensor comprises at least two sensors, wherein at least one of thesensors senses activity level and at least one of the sensors sensesbody position.
 17. The device of claim 14, wherein the at least onesensor comprises at least two sensors, wherein each of the at least twosensors is operative to sense activity level and body position.
 18. Thedevice of claim 14, wherein the controller is further operative todetermine a stimulation rate based upon activity level and bodyposition.